This invention pertains to methods and apparatus for cardiac rhythm management. In particular, the invention relates to methods and apparatus for providing ventricular resynchronization therapy.
Cardiac rhythm management devices are implantable devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm. A pacemaker, for example, is a cardiac rhythm management device that paces the heart with timed pacing pulses. The most common condition for which pacemakers are used is in the treatment of bradycardia, where the ventricular rate is too slow. Atrio-ventricular conduction defects (i.e., AV block) that are fixed or intermittent and sick sinus syndrome represent the most common causes of bradycardia for which permanent pacing may be indicated. If functioning properly, the pacemaker makes up for the heart""s inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate. Pacing therapy may also be applied in order to treat cardiac rhythms that are too fast, termed anti-tachycardia pacing. (As the term is used herein, a pacemaker is any cardiac rhythm management device with a pacing functionality, regardless of any other functions it may perform such as cardioversion or defibrillation.)
Also included within the concept of cardiac rhythm is the degree to which the heart chambers contract in a coordinated manner during a cardiac cycle to result in the efficient pumping of blood. The heart has specialized conduction pathways in both the atria and the ventricles that enable the rapid conduction of excitation (i.e., depolarization) throughout the myocardium. These pathways conduct excitatory impulses from the sino-atrial node to the atrial myocardium, to the atrio-ventricular node, and thence to the ventricular myocardium to result in a coordinated contraction of both atria and both ventricles. This both synchronizes the contractions of the muscle fibers of each chamber and synchronizes the contraction of each atrium or ventricle with the contralateral atrium or ventricle. Without the synchronization afforded by the normally functioning specialized conduction pathways, the heart""s pumping efficiency is greatly diminished. Patients who exhibit pathology of these conduction pathways, such as bundle branch blocks, can thus suffer compromised cardiac output.
Patients with conventional pacemakers can also have compromised cardiac output because artificial pacing with an electrode fixed into an area of the myocardium does not take advantage of the above-described specialized conduction system. This is because the specialized conduction system can only be entered by impulses emanating from the sino-atrial or atrio-ventricular nodes. The spread of excitation from a single pacing site must proceed only via the much slower conducting muscle fibers of either the atria or the ventricles, resulting in the part of the myocardium stimulated by the pacing electrode contracting well before parts of the chamber located more distally to the electrode, including the myocardium of the chamber contralateral to the pacing site. Although the pumping efficiency of the heart is somewhat reduced from the optimum, most patients can still maintain more than adequate cardiac output with artificial pacing.
Heart failure is clinical syndrome in which an abnormality of cardiac function causes cardiac output to fall below a level adequate to meet the metabolic demand of peripheral tissues and is usually referred to as congestive heart failure (CHF) due to the accompanying venous and pulmonary congestion. CHF can be due to a variety of etiologies with ischemic heart disease being the most common. Some CHF patients suffer from some degree of AV block or are chronotropically deficient such that their cardiac output can be improved with conventional bradycardia pacing. Such pacing, however, may result in some degree of uncoordination in atrial and/or ventricular contractions due to the way in which pacing excitation is spread throughout the myocardium as described above. The resulting diminishment in cardiac output may be significant in a CHF patient whose cardiac output is already compromised. Intraventricular and/or interventricular conduction defects (e.g., bundle branch blocks) are also commonly found in CHF patients. In order to treat these problems, cardiac rhythm management devices have been developed which provide pacing stimulation to one or more heart chambers in an attempt to improve the coordination of atrial and/or ventricular contractions, termed cardiac resynchronization therapy.
Cardiac resynchronization therapy can most conveniently be delivered by a cardiac rhythm management device in accordance with a bradycardia pacing mode so that the selected heart chambers are both resynchronized and paced simultaneously. One heart chamber, designated as the rate chamber, is paced with the bradycardia mode while the the contralateral chamber, designated as the synchronized chamber, is paced with a synchronized mode based upon senses and paces occurring in the rate chamber. In order to protect the synchronized chamber from being paced near the time of an intrinsic contraction and within its vulnerable period, a synchronized chamber protection period is initiated by a sense or pace in the chamber which inhibits any scheduled synchronized pace.
In order for cardiac resynchronization therapy to be effective, resynchronization pacing pulses should be delivered as often as possible. The present invention is directed toward a method that attempts to maintain resychronization pacing when it would otherwise be inhibited by the synchronized chamber protective period. Accordingly, a pace to the synchronized chamber is delayed by a protective delay interval when the scheduled pace would otherwise occur within the synchronized chamber protection period.